Jojo's Bizarre Adventure. one of the longest ongoing series ever and its still going. since 1987 :3
its JUMPs 2nd longest running manga
honestly my fav and i think the best classic ever. id hate anyone who dissed JJBA

As far as a series that's complete, Maison Ikkoku, Happy Mania, Yu Yu Hakusho, and Dragonball top my list. Series that I love but that have not yet finished: Berserk, Blade of the Immortal, HunterxHunter, Saiyuki, and Kurosagi Corpse Delivery Service.

finished: eyeshield 21, fruits basket, psyren, and school rumble(even tho the pages weren't a lot)
currently reading: skip beat, one piece, beelzebub, sket dance, until death do us part, gakuen alice,, and hajime no ippo(insanely long, kinda stopped reading but will pick up)

i only posted manga with over 100 chapters but i've finished and i'm currently reading other long manga

Manga I own: Chibi Vampire. (I think 15 Volumes is more than enough). Overall Manga: One Piece. (Reading Online. There's no way I'm buying over 60 volumes [$10.95 * 62 + 8.25% = way more than I can afford...]).